This application claims priority, under 35 USC xc2xa7 119, to French Patent Application No. 01 14 164, filed Oct. 31, 2001.
1. Field of the Invention
The field of the present invention is that of semiconductor optical components, to be more precise semiconductor lasers.
2. Description of the Related Art
As is known in the art, semiconductor lasers are used in many applications and in particular for pumping solid state lasers or amplifiers consisting of optical fibers doped with rare earths. Semiconductor lasers comprise an active layer based on arsenic, gallium and/or phosphorus and indium. When supplied with a current made up of p-type and n-type charge carriers, the active layer emits laser radiation which can be amplified and at wavelengths that are generally in a band around 808 nm, 920 nm, 940 nm or 1480 nm, and in particular around 980 nm, the latter wavelength being used to pump monomode fiber amplifiers in optical telecommunication applications.
The above kind of laser, which is often of the rectangular block type, can have front and rear lateral faces cleaved to form faceted mirrors so that longitudinal Fabry-Pxc3xa9rot propagation modes are established in the laser: this type of component exhibits multimode behavior in the spectral domain and monomode behavior in the TE (transverse electric) spatial domain.
Throughout the remainder of this description, the term xe2x80x9clayerxe2x80x9d may refer to a single layer or to a stack of layers having the same function.
High-power semiconductor lasers provide a high output optical power. Their performance increases as internal losses and vertical divergence of the laser radiation at the output are reduced, in particular to optimize the rate of coupling into an optical fiber.
The vertical divergence at the output is reduced by modifying the optical mode guidance properties, for example, and especially by increasing optical mode vertical spreading, which increases internal losses. Because these two parameters are interrelated, a compromise must generally be found to enable them to be adjusted at the same time.
U.S. Pat. No. 5,594,749 discloses a semiconductor laser designed to have a low vertical far field (VFF), i.e. a divergence less than 30xc2x0. In one of the embodiments described therein, the laser comprises the following successive layers:
an n-doped Al0.40Ga0.60As bottom cladding layer 2.5 xcexcm thick,
an undoped AlxGa1-xAs bottom barrier layer, with x varying from 0.40 to 0.25 and having a linearly varying refractive index, i.e. a graded index (GRIN) layer, 100 nm thick,
an undoped GaAs active layer with quantum wells, 7 nm thick,
an undoped graded index AlxGa1-xAs top barrier layer, with x varying from 0.25 to 0.40 and 100 nm thick, the last three layers being included in a light guide region 0.27 xcexcm thick, and
a p-doped Al0.40Ga0.60As top cladding layer 1.85 xcexcm thick.
Performance in terms of divergence and internal losses (as indicated by the radiation absorption coefficient (expressed in cmxe2x88x921) in the lossy regions) as a function of the chosen thickness of the doped cladding layers is shown in the form of graphs.
The graphs show that, for a given thickness, divergence decreases and absorption increases as the aluminum ratio decreases.
In all the embodiments described, the light guide of the laser forms a thin undoped central region and the internal losses are therefore not totally compensated, because low divergence is obtained at the cost of a non-negligible increase in internal losses.
An object of the present invention is to provide a semiconductor laser having minimum internal losses combined with minimum vertical divergence and based on a vertical semiconductor layer structure that is simple and easy to produce.
To this end, the present invention proposes a vertical structure semiconductor laser comprising:
an n-type doped semiconductor bottom cladding layer,
a light guide superposed on the bottom cladding layer and including a central region including an semiconductor active layer, and
a p-type doped semiconductor top cladding layer superposed on said light guide,
which laser is characterized in that said light guide further comprises:
a semiconductor bottom guide layer having the following two adjacent bottom parts:
an undoped first bottom part adjacent the central region, and
an n-type doped second bottom part adjacent the bottom cladding layer,
a semiconductor top guide layer having the following two adjacent top parts:
an undoped first top part adjacent the central region, and
a p-type doped second top part adjacent the top cladding layer,
in that the first bottom and top parts form a non-doped region more than 0.5 xcexcm thick, and in that the refractive index difference between one or each cladding layer and the adjacent guide layer is less than 0.02.
In accordance with the invention, because of the insertion of the guide layers, the light guide is much thicker than that of prior art lasers. The resulting vertical spreading of the optical mode produces low vertical divergence at the output of the laser of the invention.
Moreover, the low (or even zero) index step between the cladding layer and the adjacent guide layer helps to reduce the divergence.
With regard to internal losses, it is known in the art that one cause of increased internal losses in a laser is interaction between the n-type or p-type charge carriers and the electromagnetic field of the optical mode. In the invention, to limit the penetration of the field into the doped areas, the first parts of the guide layers are not doped. The internal losses are therefore of the same order as those of a thinner light guide with a high rate of confinement of the electromagnetic field and greater divergence.
In the invention, the semiconductor layers do not install barriers or potential wells on the path of the n-type or p-type charge carriers. The electrical behavior and the thermal behavior of a laser of the invention are therefore satisfactory.
A high-power laser generally operates at a current equal to approximately ten times the threshold current. The threshold current of a laser of the invention can easily be kept to a level compatible with high-power operation.
The thickness of the non-doped region (ND) can advantageously be substantially equal to 0.8 xcexcm. In the invention, the combined thickness of the bottom and top guide layers can be from 1.2 xcexcm to 1.5 xcexcm.
There is therefore a limit on guide layer thickness so that their contribution to the series resistance of the laser of the invention remains acceptable and they do not encourage the appearance of higher transverse modes likely to produce laser radiation.
In an advantageous embodiment, the refractive index difference between a cladding layer and the adjacent guide layer is substantially equal to zero.
This minimizes the divergence.
In the invention, the bottom and top guide layers can be of the same semiconductor material and preferably of a compound containing arsenic, gallium and aluminum with an aluminum atomic ratio from 0.25 to 0.30.
A compound of the above type with an aluminum atomic ratio of 0.27, denoted Al0.27Ga0.73As, gives a refractive index approximately equal to 3.37. For example, the cladding layers can be of Al0.30Ga0.70As, giving an index step of 0.016.
Moreover, the central region can comprise:
an undoped semiconductor bottom barrier layer between the first bottom part and the active layer, and
an undoped semiconductor top barrier layer between the first top part and the active layer,
and, according to the invention, the top and bottom barrier layers can be of the same semiconductor material, for a simpler structure, and preferably of the same compound containing arsenic, gallium and aluminum.
The higher the aluminum atomic ratio, the lower the refractive index. A high refractive index in the vicinity of the active layer provides sufficient optical confinement.